Approximately 20-30% of patients with focal epilepsy are medically refractory to treatment with anti-epileptic drugs. These patients are potential candidates for curative respective surgery [1, 2]. The primary aim of epilepsy surgery is to remove the epileptogenic zone: “the minimum amount of cortex that must be resected (inactivated or completely disconnected) to produce seizure freedom” [3, 4]. The identification of the epileptogenic zone usually involves the placement of intracranial electrodes to record where seizures start and rapidly propagate. Stereo-electroencephalography (SEEG) is the practice of recording electroencephalographic signals via depth electrodes that are surgically implanted into the brain tissue. A significant challenge in epilepsy surgery now is in the treatment of the more difficult patient groups (extratemporal non-lesional), where SEEG is increasingly utilised. This invasive investigation carries the risks of infection, haemorrhage and neurological deficit [5].
Preoperative planning of SEEG electrode placement is a necessary prerequisite to implantation. Important anatomical and functional landmarks of the brain (such as blood vessels, pial boundaries, nerve tracts, etc.) can be identified with advanced neuro-imaging and image-processing techniques. SEEG electrode trajectories are defined by a target area that has to be reached by the electrode and an entry point where the electrode penetrates the skull. Electrode arrangements are planned to achieve adequate cortical coverage and pass through safe, avascular planes. The large number of electrodes required in SEEG and the cumulative risk associated with this implies that computer-assisted planning (AP) would be very useful in such clinical cases.
Previous work on pre-operative planning of depth electrode placement describes approaches to find the optimal path either automatically [6, 7, 8] or by assisting the decision-making process of the neurosurgeon [9, 10, 11]. Another approach [12] has proposed a system to assist at all stages of the planning from the selection of the target point to the selection of a safe entry point that minimizes the risk of hitting a vital structure. In all of these approaches, the operator selects the target point precisely and the time required to compute the optimized paths is generally long. Recently, a high performance solution has been described to enable quantitative estimation of the risk associated with a particular access path at interactive rates [13]. In this approach, graphics processing units (GPUs) are employed to achieve real-time speed, and a custom form of visualization (risk map) is used to aid the planning process.
Nevertheless, the pre-operative planning of electrode placement for stereo-electroencephalography and other such intra-cranial procedures is still frequently viewed as a relatively difficult and time-consuming process.